Heat treating furnace



r/l/A 4 Sheets-Sheet 3 s. K. WELLMAN HEAT TREATING FURNACE Filed July 1,1959 Oct. 7, 1941.

Patented Oct. 7,` 1941 UNITED STATES PATENT OFFICE HEAT TREATIN GFURNACE Samuel K. Wellman, Cleveland Heights, Ohio, assignor to The S.K. Wellman Company, Cleveland, Ohio, a corporation of Ohio Thisinvention relates to heat treating apparatus and particularly to furnaceapparatus suitable for welding together the parts of composite metallicarticles.

The present application constitutes a continuation in part of my earlierapplication Serial No. 210,651, filed May 28, 1938, upon which PatentNo. 2,178,527 was granted October 31, 1939, and in which I havedisclosed'my improved apparatus as adapted for the heat treatment ofcomposite machine elements for friction clutches, friction brakes,bearings and other devices in which one element slidably engages anotherelement or body, the particular machine elements referred to each havinga backing member of ferrous metal and weldedl thereto a facing mem.-

' ber of compressed powdered material consisting predominantly ofrelatively high-melting-point .metal such as copper or iron.Accordingly,` in

the present applicationmy improved apparatus is described as applied tothe production of such machine elements and specifically friction clutchdisks or plates.

One of the objects of the present invention is the provision of a heattreating furnace adapted Application July 1, 1939, Serial No. 282,344

to subject the 4work rduring treatment to pressure 1 which iscontrollable at Will.

Another object of the invention is the provision of an electricallyheated furnace of simple and rugged construction in which the work undertreatment can be effectively subjected to selected atmospheres.

A further object of the invention is the proance with the presentinvention.

struction which will be pointed out in the following descriptionofspecific furnaces shown in the accompanying drawings.

In the drawings,

Fig. lis a central vertical section of a'sintering and Welding furnaceconstructed in accordj Fig. 2 is an enlarged fragmentary sectionalview'showing a portion of the structure shown in Fig. 1.

Fig. 3 is a view similar to Fig. 2 butshowing Fig. 8 is a centralvertical sectional view of the. furnace shown in Fig. 1- with some ofits interior fittings modified to adapt the furnace for a modified formof heat treatment.

Fig. 9 is -a horizontal sectional view taken on the-une s-s f Fig. s.

vision of a heat treating furnace adapted to subject the work duringtreatment to pressure and having an effective air seal between the mainwalls of the furnace and the work under treatment.

Another object of the invention is the provision .of a furnace having alchamber for the work to be treated, a second chamber surrounding thework chamber and means for introducing selected atmospheres into therespective chambers. l

Still another object of the invention is the provision of improved Workseparators for the furnace whereby a plurality of composite elements tobe treated can be stacked up with separators interposed between them andsubjected to simultaneous pressure and heating without danger of thedifferent elements being Fig. 10 is a vertical sectional view of a modi-.fi'ed construction of the furnace cover or container shown in Figs. 1to 9,'inclusive, that portion of the figure above the metal plate lc-Ibeing taken on the line ifi-I0 of Fig. 1\1 and .the portion below platek-I being taken on the line |0'-|0 of Fig. 11.

Fig. 11 is a plan view of the modified furnace cover construction shownin Fig. 10, with parts broken away to show a portion of the cover insection on the plane of Fig. 10. v

Fig. 12 is an enlarged fragmentary vertical indicated by the line II-II'sectional view of parts shown in Fig. 10. y

Fig. 13 is a vertical sectional view on the line iSv-i3 of Fig. l2.

Fig. 14 is a horizontal sectional the line H-M of Fig. 12.

Referring iny detail to the construction-shown in Figs. 1 to 7, thefurnace comprises a base structure designated asan entirety by theletter view taken on a and a cover or container structure designated asan entirety by the letter b. The base structure comprises a pair ofunderlying channel members a-l, a-l upon which is mounted and secured(as by welding) a sheet steel tub or pan a-2 with the top of its sidewall anged outward at af-3. The bottom oi' pan a.-2 has a heatinsulating lining afp-4 which is formed with a cen' tral cylindricalupward extension a-5 designed to support the work in the furnace.Extension a-5 is covered by an iron casting a-6 which is cored out toform top and bottom plates af-l, a--8 connected by vertical webs a-9,thus providing pa-ssageways for the circulation of gases which may beintroduced through pipe a,-I0. The horizontal branch of pipe af-II'I isfitted with a. downwardly opening nipple af--I I, the purpose of whichwill later be explained. Branch pipe a-'|2 joined to the-upright sectionof pipe .a-IU serves as a conduit for thermocouple conductors, as willalso later be explained.

The furnace cover structure b comprises an inverted cup-shape casingstructure of sheet steel which consists of a circular top plate b-I, acylindrical side wall b--2 attached to the top wall b--I by bolts b-3,and annular bottom wall b-4 secured to side wall b-2 by bolts b-B andhaving a depending flange or skirt b-B.

This casing structure is .strengthened by a plurality of tie rods orbolts b-l joining top plate b--I and bottom plate b-4. The coverstructure b has its inner side wall formed by a tubular electric heatingelement b-B which consists of a wall b-S of refractory and heatinsulating cementin which is embedded an electric helical heating coilb-I, the terminals of this coil being shown at b--II, b-IZ. The open topof heating element b--B is closed by a sheet metal plate b-I3 and thespace between the latter and the top plate b-I is filled with heatinsulating material b-IL .Also thel space between side wall b-2` andheating element b--B is filled with loose heat insulating material b-I5.

For the purpose of applying downward pressure to the furnace cover, thelatter is fitted with an air pressure device c which consists of an airchamber c--I formed by top and bottom circular 4 of heavy canvass, theedges of the circular plates c-2, the rubber wall c--3 and the canvassstrips being hermetically joined as by bolts 0 5. The air chamber c-Irests upon and is secured, as by welding, to a pair. of steel channelsc-6, c8 which are in turn welded to the top of the furnace cover (Fig.'7). To the top of the air chamber is welded a circular reinforcingplate c7 on which in turn is welded a pair of channel members c-8. Anapertured plate c-9 joins channels c-8 and a hoist may be attached tothis plate c-9 to lift the furnace cover.

The base structure of the furnace carries a number of upstanding tiebolts or rods a-I3 which slidably engage guides c-III carried by channelmembers c-S and guides.c-II carried by channels c-8. Thus, when the nutsa-H of tie rods a-I3 are screwed down on channels c-B a top abutment isafforded for the air chamy ber o-I, enabling said chamber when inflatedto subject the top 4of the furnace to downward pressure. The top plateof air chamber c-I is provided with a'valve-controlled air inlet c-I2,also with an air relief valve c--I3 and a pressure gauge c--IL In theuse of the furnace in carrying out the process or procedure lastreferred to. the work to be welded is enclosed within a tightly sealedmetal chamberor container within the furnace. This inner sealed chamber'is designated as an 7 entirety by the letter d and comprises a metalbase df-I (which in effect constitutes' an upward extension of thecentral part of base a), a cylindrical side wall d--2 and a sheet metaltop wall or diaphragm (if-3. The side wall d-2 is tightly secured orsealed by bolts as shown to the base part d-I Vand diaphragm d-3 istightly secured to the side wall by a bolted ring d-4. A pressure blockd--5 rests upon diaphragm d-3.

In order to provide for. the maintenance of a current of suitablereducing gas through the sealed chamber d, the base plate d-I is fittedwith an inletu tube d-S and an outlet tube d-l, said tubes being bent asshown in Fig. 1 to pass under the side wall of the furnace cover.

The sealed chamber d is provided with a protecting air seal in the formof an inverted cupshape cover or container e of sheet metal, the topwall of this cover being interposed between 4the block d-S and thetopportion of the furnace cover b.

With the parts of the furnace assembled as shown in Fig. l, thedepending skirt b-6 of the furnace cover and the lower end of the airseal cover e are embedded in loose heat insulating material a-IS in thefurnace base so as to prevent Ifree access of air to the interior of thefurnace `chambers. In Fig. l thermocouples are shown at f-I and f2 incontact with the base and side wall, respectively, of the sealed chamberd, the conductors for t ese thermocouples being carried to the exter orof the furnace through pipes a-III and v4-I2.

In Figs. 1, 2, 3 and 5 the work to be treated in the furnace is shown atg within the sealed chamber d and consists of a stack of compositeclutch disks I, I together with suitably interposed separatorsh betweenthe individual clutch disk assemblies, the separators serving to preventthe welding together of separate clutch disks or assemblies during theheat treatment.

Each of the clutch disks I, I comprises a disk proper Ia (Figs. 2 and 3)which is preferably formed of sheet steel, and two facing rings Ib, Ibwhich are formed of highly compressed and coherent powdered frictionmaterial of the character stated in the aforementioned applicationSerial No. 210,651 and consisting predominantly of high-melting-pointmetal such as copper orl iron or mixtures of copper and iron andpreferably also containing one or more additional metals with or withoutsome solid, non-metallic material adapted to decrease or increase thecoefficient o f friction of the resulting mixture. For clutch and brakedisks suitable for many uses a satisfactory composition is '73% copper,14% lead,' '7% tin, and 6% graphite. For friction clutches, there mayalso be used Isatisfactorily a composition consisting of '70.9% copper,10.9% lead, 6.3% tin, '7.4% graphite and 4.5% air floated silica; also acomposition consisting of 62% copper, 8% iron, 12% lead, '7% tin, '7%graphite and 4% silica.

' Each of the separators h consists of a steel disk h-I similar to thedisks la, to which facings 71,-2 and h 3 of compressed and coherentpowdered material which may be of the same character as the facings Ibof the clutch disks -I, have previously been welded, together with athin coating h/-l of graphite on the exposed face of each compressedpowdered part, the graphite in practice being sprayed in colloidal stateupon the separator to form, when dried, a thin even coating. 'I'heseseparators are remarkably-strong and their faces formed by the porousfacing material, before they are coated with graphite, can be ground toaccurate parallelism so vthat the parts of the individual clutch diskassemblies may be subjected to uniform pressure throughout theirrespective areas by the applica- .very durable. f

The specific heat treatment to be carried out 'in the furnace in thecase of the composite clutch disks is the welding of the porous facingmembers Ib of said disks to the steel disks or backing members la. As isexplained in the aforesaid application Serial No. 210,651, the porousfacing members tend to sorb gases and vapors capable of corrodingferrous metals and prior to my invention no one to my knowledge hadsucceeded in satisfactorily welding such facing members to ferrous metalbacking members because the gases or' vapors sorbed 'in the facingmembers were driven out during the welding operation against the ferrousmetal of 'thebacking member and oxidized or corroded the latter in amanner pref venting the formation of a good weld or bond. The improvedfurnace, as shown in Figs. 1 to 7, is designed to overcome thisdiiiculty by permitting the .elimination ofv moisture or corrosive gasesfrom the compressed powdered facing members in a manner' preventingcorrosion of the ferrous backing member, and thereafter effecting thewelding together of the facing and backing members. i

In the use of the furnace the'facing members ib, ib, as assembledtherein, may be either sintered or unsintered, but in the presentdescriptionit is assumed that they have not previously been sintered. Tocarry out the procedure of eliminating moisture or gases from the porousfacing members, the sintering of said members and the welding of saidfacing members to the steel backing members, the top of the cover orouter container of the furnace and the air seal structure e having firstbeen lifted olf the furnace base, the work to be treated is ass'embledin the chamber d and the latter closed I by bolting on the top diaphragmd/-3 thereof as side and top walls of chamber 4d can then be bolted toplate d-L Next, the pressure block d-5 is applied, the air seal e isplaced over the chamber d and the cover `of the furnace is lowered intoposition over the furnace base ,so that the skirt b-S is embedded in theloose insulation o--IS but with the furnace top or cover supported bythe' hoist. Under these conditions the stack of work is `not subjectedto pressure,

lexcept for the moderate weight of the work itself.

4As heating of the work in the furnace is started a current of dryreducing gas, preferably hydrogen, is started flowing through'inlet tubed-6 into the interior ofthe sealed chamber d andout from said chamberthrough tube d-J, outlet tube Vdf-l being controlled y(by water seal@-3, for example) so as `toregulate the flow and maintain asuperatmospheric pressure in chamber d. At the same time that the flowof reducing gas is started through tube d--6, a supply of non-oxidizinggas, such as natural gas or the applied to ignite such gas as escapesthrough the nipple a-II of the pipe, the llame at this point serving toindicate to the operator that the combustible gas is entering that partof\the furnace chamber between the sealed chamber d and the airprotecting seal e and maintaining' l a suillcient pressure to preventinfiltration of air. This is accomplished without subjecting the heatingelement b-B of the furnace'to the action of non-oxidizing or reducinggases.

The dry hydrogen introduced as aforesaid into chamber d diffusestherein, permeating the interstices between the assemblies of thestacked-up work, and serves; if its flow is sufficiently rapid, toabsorb and carry out from thesealed chamber vapors and gases expelledfrom the pores of the facing material substantially as rapidly as theyare expelled by the heat of 'the furnace.- Accordingly,`as the furnaceis heated up to the welding temperature any moisture and gases drivenout of the porous facing members is prevented from having corrosiveaction upon the surfaces of the ferrous backing members. As thetemperature of the furnace is gradually increased 1A and reaches a valueof 1200 to 1300 F.,`sintering of the compressed powdered facings iseffected. With further heating, the welding temperature lying in therange of 1350 to 1500 F. is reached,

thehoist supporting the top of the furnace 'is eased off, nuts af-M areapplie'd to the tie rods (1f-I3 and the pressure device' c--I isinflated to apply downward pressure .to the furnace top b. 'Ihispressure causes the pressure block d-5 to force diaphragm d-3 downward,as shown in Figs. 1 and 2, against the stack of work and applysufficient pressure theretoto insure firm` contact between thecompressed powdered facings and the clutch disks throughout the 'entireextent of their contiguous surfaces and thus effect the desired weldbetween the parts.

yWhen the weld has been effected by the application of pressure as abovedescribed, the top of the furnace is lifted off and, with the protectiveair seal e remaining lin positi0n, the parts of the furnace,iincludingthe sealed chamberv d, and the welded work are allowed to cool, thepressure of the hydrogenin the sealed chamber d and of the natural gasin the space surrounding .the chamber d being meanwhile kept slightly,v

above atmospheric pressure until the chamberfd and the work therein havereached a sufficiently low temperature to prevent oxidation in contactwith the air, whereupon the protective seal e can be removed and thechamber -d further cooled to permit itV to be opened for the removal4member of the clutch disk can be effectively protected from corrodingor oxidizing action ofmoisture or gases sorbed in the facing andsuccessful welding of the porous facing directly to the backing memberbe attained consists in applying tothe cleaned surface of the backingmember a film of a suitable material capable of protecting the-backingmember from the corroding or oxidizing action without interfering withHowever, it is relatively inexpensive and the formation of a direct weldbetween the ferrous backing member and the predominant highmelting-pointmetal or metals of the facing member. Such a protecting film can beformed by the electric deposition of suitable metals on the backingmember. A coating of copper is suitable where the predominanthigh-meltingpoint metal of the facing consists entirely of copper, Whilea coating of nickel is suitable where the predominant high-melting-pointmetal of the facing member includes a substantial percentage of ironpowder. Such. a coating of nickel is preferably applied over a .thinplating of copper on the ferrous backing member, in accordance withusual nickel plating practice.

In the use of the protective metal coating, the assembled electroplatedbacking members and compressed facing members may be welded in a furnaceof the type shown in Figs. 1 to 7, though with the use of the coatedbacking members the inner diaphragm chamber of the furnace becomesunnecessary. In Figs. 8 and.9 the modifled furnace is shown with acharge therein of the assembled coated backing members and facingmembers, together with separators of the character already described.

With the omission of the sealed inner chamber d the stack of work to beweldedis placed direct- \ly upon the base casting a-G while a similarcored out casting i is placed upon the top of the stack of work and-oncasting i in turn is placed a block y of heat insulating refractorymaterial to be engaged by the top of the air sealing structure e of thefurnace.

The welding operation, or combined sintering and welding operation, iscarried out as follows. With the furnace top removed, the Work to bewelded is placed upon the base casting a-B of the furnace, charcoal isplaced on the casting a.-6 around the work as indicated at k, theprotective structure e is placed over the work, the

furnace top is lowered into position and the air.

pressure device inflated to apply sufficient pressure to the elements tobe welded to insure their rm mutual contact. Thereupon heating of thefurnace is started and as its `temperature rises lair initially in thefurnace oxidizes the charcoal at 1c to form carbon monoxide which fillsthe furnace chamber. Meanwhile pipe a-IU can be opened to the supply ofnatural gas which, as in the last described procedure, prevents entranceof air into the furnace while it is cooling.

As the temperature of the furnace is raised 'any moisture Aor oxidizingor corroding gases sorbed in the compressed POwdered facings will bedriven out by the time the temperature has reached about 750 F. but thethin coating of copper or nickel on the ferrous backing members protectsthe latter from oxidizing or corroding action by these vapors or gases.

When the temperature of the furnace rises inmelting-point metal of thepowdered facing material, higher temperatures than th'ose given abovemay be required to effect sintering andl welding, particularly wherevcarbon monoxide is used as reducing gas in the furnace.

In Figs. 10 to 14, inclusive, is shown a modification of the outer coveror container structure ofthe furnace. housing. In this modification theside wall of the cover or container has the tubular heating coilseparably mounted so that it can g readily be replaced without thenecessity of disto the range of 1200 to 1300 F. sintering of thecompressed powdered facing is effected and with furtherv rise of thetemperature within the range of 13501500 F. the facings become welded tothe steel clutch plates. Thereupon the furnace top is removed, the airseal e being meanwhile left in position until the work has had time tocool and the supply of reducing gas also being maintained through thepipe af-lll meanwhile to prevent infiltration of air to the work whileit is cooling.

With respect to the welding procedures which `have been described aboveit may be observed that if iron constitutes a large part of thehighmantling or tearing down the entire furnace.

According to this construction, the cover consists of a circular topplate lQ-I, a cylindrical metal sidewall lc-2 attached to the top platek-I by means of bolts Ilz-3, an annular bottom Wall composed of a pairof metal rings Ic-4 and k-B secured to side wall Io-Z by bolts Io-G, andhaving a depending flange or skirt lc-l. Ring lo-d may be composed ofany convenient ferrous metal, while ring 1:-5 should preferably becomposed of heat resisting steel of the stainless type. 'A tubularretaining wall lc--8, preferably of refractory material, is disposedconcentrically on ring lc-d, the retaining wall having about the sameinner diameter as that of ring lc-l. A plurality of brick columns lo-9,k-9 are disposed on ring k--ll between retaining wall Io--8 and sidewall k 2 and are spaced approximately uniformly apart. These columnsextend upwardly to the top of the retaining wall k-B and serve tosupport a heat resistant metal plate kel!) which in turn supports rigidrefractory heat insulating material k-II disposed thereon to fill thespace between plate Io-I and top plate k-I Loose heat insulatingmaterial lc-IZ fllls the remaining space bounded by bottom Wall k-4,side Wall Ic-2, retaining wall lo-, and top plate k-|. A plurality oftie rods 7f3-'I3 extend between the top plate and bottom rings, eachhaving nuts threadably engaged with opposite ends and adapted to betightened against top plate Ic-l and bottom ring Ic-5 so as to draw thewhole structure tightly together, thereby making a rigid,self-sustaining unit yet not plac- `ing any substantial endwise pressureon retaining Wall Ic-B. By avoiding such pressure on the retaining wall,bulging and distortion of the wall are also avoided when it is heated upto the high temperaturas attained in the furnace.

The rigid unit thus produced acts as insulation and support for a.replaceable inner wall and heating element k--i4, which is formed in theshape of a tube having an outer diameter slightly smaller than the innerdiameter of retaining wall 1x1- 8. The tubular member is thus adapted tobe slipped inside the retaining wall. 'I'he tubular structure consistsof a body k-l 5 of rigid refractory heat insulating material in which isembedded an electric heating coil Io-I terminating in connector blocksIo-ll and k-IB.

k-ZI, Ie--2I.V Fectrical connections are made' with the heating Vcoil bymeans of rods k-22 which extend through insulating bushings k-24,'7c-24, which pass through side wall Ic-Z, insulating material itz-I2,and holes lc-23 in retaining wall 1o-8. Rods k-22 are reduced indiameter and threaded at ,their inner ends' and are adapted to bescrewed into terminal blocks lc-I1 and lc-IB. Leads toan electriccircuit may be connected to their outer endsby means of clips or clampslc-25.

The structure of blocks k-Il land Ic'-l8 will be understood more fullythrough reference to Figs. 12, 13 and 14. They consist of solid blocksof heat resisting metal, such as stainless steel, which are about halfas thick as wall Ic-I5,

and are embedded in the wall adjacent opposite ends of a longitudinalelement thereof with their outer faces approximately iiush with theouter surface of wall vlc---I5. Each block has a longitudinal lholelc-ZS extending therethrough near the bottom of top block k-Il and nearthe top of bottom block 1c-18, the hole being large enough to receivethe resistance wire forming coil 7c l6.

' Set screws iff-21 are threaded into the blocks so It will be seen thatthe furnace'cover or con-4 tainer as constructed according to theforegoing cover, may exert sufficient pressure on the work. This, ofcourse, is more likely to be the case when the innermost container d isnot used.

While the forms of construction which have been shown and described aresuch as are preferred, it should be understood that variousmodiiications of the constructions disclosed may be made within thebounds of the invention as defined in the appended claims'.

1. In a heat treating furnace, the combination of .means for supportingan upright stack of plate-like articles to be'treated; housing meanscomprising a base structure and a rigid, unitary, bell-like cover havinga top wall and a side wall depending therefrom, the said top and sidewalls in the operation of the furnace being respectively disposedaboveand laterally around the stack of articles' under treatment;sealing means cooperating with the cover side wall and the basestructure to prevent free escape of gas from the interior of thehousing, said sealing means being constructed and arranged to permit thesaid cover to be raised away from the base structure; means carried bythe said'cover for heating the chamber enclosed by the cover; and fluidpressure Imeans operable at will for compressing the stack of articlesunder treatment between the support modification permits the heatingelement tobe Yremoved and replaced whenever necessary with a minimum ofdelay, while also avoiding the exy pense involved in rebuilding theinsulating struce ture each timethat a new element is installed. Forexample, in the replacement it is merely necessary to unscrew rodsIc--22 and pull them backward'out of blocks kFIl and Ic-lii. Screws4lc---ZI are then removed, whereupon flanged ring lc-l9 and the wallsection lc-ll may be lowered away from the restof the cover. The newwall section and heating element may then be inserted and flanged ringlo-l9 replaced and fastened in with screws Ic-ZL after which rodsIla-2,2 are reinserted and screwed down tight against the blocks Ic-I'Iand lc-l8.

It willbe seen that in each of the illustrated forms of the improvedheat treating furnace the apparatus comprises a base structure adaptedto support the work to be treated and a housing structure to enclose thestacked--up work, the housing means in each instance comprising one ormore covers or containers having an upper end l wall to be disposedabove the top ofthe work and construction, however, I prefer to providealso the inverted cup-shape air seal which also can be considered assuch a container.'

While in most instances' the furnace is provided with special means forforcing the outer container downward to exert pressure upon the stack ofarticles being treated, the weight of the furnaceA container orcontainers, with .or without the placing of additional weights on top ofthe furnace thereof andthe top wall of the said cover with a variablepressure in excess of the weight of the said cover and parts sustainedbyit.

2. A furnace as claimed in claim l in which the base structure of thehousing has a central elevated table constituting the support for theplate- 4. A furnace as claimedin claim 1 in which a portion of the basestructure of the housing constitutes the support for the plate-likearticles and the fluid pressure means is constructed and arranged toapply downward pressure to the top wall of the housing cover andcomprises a frame structure attached to the base structure of thehousing to form an abutment overlying the top' wall of the housing coverto take the reaction of the pressure means.

5. A furnace as claimed in claim 1 in which the housing cover comprisesa top wall section of refractory material and a side wall section ofrefractory material and the heating means of the furnace comprises anelectrical conductor embedded in the refractory material of the saidside wall.

6. A furnace as claimed in claim 1 in which th top wall of the housingcover is substantially rigidv under pressure transmitted between itsouter surface and its inner surface and comprises outer and inner metalsections and anintermediate section of refractory material, the sidewall of the said cover has a tubular inner section of rigid refractorymaterial, a tubular outer section of metal, an annular bottom metalsection and loose refractory material between the said inner and outersections. and the heating means of the furnace comprises a helicalelectrical conductor embedded in the inner, side wall section of rigidrefractory material.

7. A furnace as claimed in claim 1 in which the side wall of the housingcover comprises a tubular inner section of rigid refractory material andmeans for disengageably securing said inner section to the coverstructure so it can be removed as a unit without tearing down the coverstructure, and the heating means of the furnace comprises an electricalconductor embedded in the said inner section and terminal conductorssupported by the cover and disengageably secured to the embeddedconductor to facilitate removal of said tubular inner section.

8. A furnace as claimed in claim l in which the housing cover has a.substantially rigid top wall and a tubular side wall having a tubularinner section of rigid refractory material, a tubular outer section ofmetal, an annular bottom section of metal, a plurality oi' pillars ofrefractory material extending between the top wall and the bottomsection of the side wall, and a plurality of tie rods disposed betweenthe inner and outer sections of the side wall and connecting the topwall and the bottom section of the side wall to bind the parts of thecover together.

9. A furnace as claimed in claim 1 in which the housing means includesin addition to the cover an air seal of bell-like shape having the upperside of its end w'all shaped to conform to and engage the under side ofthe cover end wall and the lower part of its side wall engaging thesealing means so that the air seal encloses the stack of articles to betreated and serves following heat treatment when the cover is raised oithe base to prevent access of air to said articles while the latter arecooling.

10. A furnace as claimed in claim 1 in which the housing means includesa container disposed within the bell-like cover and having a side wallsupported on the base structure and atop end wall with a part thereofsupporting the cover and movable downward with said cover to applypressure to the stack of articles to be treated.

11. A furnace as claimed in claim l in which the bell-like cover is theouter of three housingmeans containers having in each case a top walland a sidewall, namely, an inner container, au air seal containersurrounding the inner container and the outer container surrounding theair seal container, the inner container having its side wall supportedon the base structure and its top wall deformable under downwardpressure to exert downward pressure on the stack of articles and the airseal container and outer containerhaving their side walls supported fromtheir top walls which in turn are supported by the top wall of the innercontainer, and in which there is provided means for introducing aselected gaseous atmosphere into the inner container and means.

